Certain embodiments of the present invention relate to an electron beam tomography (EBT) scanner. More particularly, certain embodiments relate to a method and apparatus for reducing variation in a spot size of an electron beam at a target due to multipole aberrations in an electron beam tomography (EBT) scanner.
EBT scanners are generally described in U.S. Pat. No. 4,352,021 to Boyd, et al. (Sep. 28, 1982), and U.S. Pat. No. 4,521,900 (Jun. 4, 1985), U.S. Pat. No. 4,521,901 (Jun. 4, 1985), U.S. Pat. No. 4,625,150 (Nov. 25, 1986), U.S. Pat. No. 4,644,168 (Feb. 17, 1987), U.S. Pat. No. 5,193,105 (Mar. 9, 1993), U.S. Pat. No. 5,289,519 (Feb. 22, 1994), U.S. Pat. No. 5,719,914 (Feb. 17, 1998) and U.S. Pat. No. 6,208,711 all to Rand, et al. Applicants refer to and incorporate herein by reference each above listed patent to Rand, et al.
As described in the above-referenced Rand et al. patents, an electron beam is produced by an electron gun at the upstream end of an evacuated, generally conical shaped housing chamber. A large negative potential (e.g. 130 kV or 140 kV) on the electron gun cathode accelerates the electron beam downstream along the chamber axis. Further downstream, a beam optical system that includes magnetic focusing, quadrupole, and deflection coils focuses and deflects the beam to scan along an X-ray producing target. The final beam spot at the X-ray producing target is smaller than that produced at the electron gun, and must be suitably sharp and free of aberrations so as not to degrade definition in the image rendered by the scanner.
The X-rays produced by the target penetrate a patient or other object and are detected by an array of detectors. The detector array, like the target, is coaxial with and defines a plane orthogonal to the scanner axis of symmetry. The output from the detector array is digitized, stored, and computer processed to produce a reconstructed X-ray video image of a portion of the object, typically an image of a patient""s anatomy.
In the chamber region upstream of the beam optical system, a diverging beam is desired and the electron beam may advantageously self-expand due to the force created by its own space-charge. By contrast, downstream from the beam optical system, a converging, self-focusing beam is desired to minimize the final beam spot at the X-ray producing target.
As the electron beam passes through the vacuum chamber, it ionizes residual or introduced gas therein, producing positive ions. The positive ions are useful in the downstream chamber region where space-charge neutralization and a converging beam are desired. But in the upstream region, unless removed by an external electrostatic field, the positive ions are trapped in the negative electron beam. The space-charge needed for the desired beam self-expansion may undesirably be neutralized, and the beam may even destabilize or collapse.
As described in U.S. Pat. Nos. 4,625,150, 5,193,105, and 5,289,519, the positive ions may be removed from the beam using a device that creates transverse electric fields and electric fields alternating in direction along the axis in the region between the electron gun and the beam-optical lens system (magnetic solenoid). Such a device is often referred to as an ion clearing electrode (ICE).
Using such transverse and/or alternating axial electric fields to remove positive ions between the electron gun and the beam optical lens system advantageously produces an electron beam that is self-repulsive (or self-defocusing) in the upstream or first region. The beam is self-attractive (or self-focusing) in the downstream or second region since ions are not removed here.
The first and second regions are traditionally segregated by a washer-shaped positive ion electrode (PIE), typically coupled to a high positive potential, e.g. up to +2.5 kV, as disclosed in U.S. Pat. Nos. 5,193,105, 5,289,419, and 5,386,445. The magnitude of the PIE potential may be used to determine the relative lengths of the upstream and downstream beam regions. Further, a suitably high PIE potential prevents ions created downstream from drifting into the upstream region.
All current EBT scanners incorporate some form of ICE terminated by a PIE or ion trap which prevents ions formed downstream of the ICE from drifting upstream. The ions are required to accumulate in the downstream beam in order to neutralize the downstream space-charge. The PIE causes a well-defined paraboloidal boundary to form between the space-charge-dominated beam in the ICE and the neutralized beam downstream. The paraboloidal boundary may be used to correct spherical aberration (focal strength varying with radius) in the beam self-focusing forces by varying the voltage applied to the PIE (see U.S. Pat. No. 5,719,914).
There are other non-linearities or aberrations in the electron beam focusing forces that cause imperfect final beam spots and which are known as multipole aberrations. In multipole aberrations, the focal strength varies with azimuthal angle as well as radius. The multipole aberrations are due to non-linear external forces applied to the beam by the electrodes, and residual ion clouds in the ICE system. In certain ICE systems such as the SPICE (U.S. Pat. No. 6,208,711), RICE (U.S. Pat. No. 5,193,105), and RICENOODLE (U.S. Pat. No. 5,289,519) systems, the predominant multipole aberration is the decapole in which the focusing forces have 5-fold symmetry. The 5-fold symmetry typically causes a variation of the beam spot width around the X-ray target with a period of 72 degrees.
A need exists to compensate for and reduce multipole aberrations of an electron beam in an EBT scanner in order to reduce variation in spot size at a target. More particularly, a need exists to compensate for and reduce the predominant decapole aberration.
An embodiment of the present invention provides an approach for reducing the effects of multipole aberrations in an electron beam of an EBT scanner.
A method is provided for reducing variation in a spot size of an electron beam at a target due to multipole aberrations in an electron beam tomography (EBT) scanner. A magnitude of a DC voltage is applied to a positive ion electrode (PIE) within the EBT scanner and is adjusted. An orientation of a non-circular aperture of the PIE is aligned with respect to the electron beam. A profile of the spot size is monitored while adjusting the magnitude of the DC voltage and while aligning the orientation of the non-circular aperture of the PIE until the variation in the spot size is sufficiently reduced.
Apparatus is also provided for reducing variation in a spot size of an electron beam at a target due to multipole aberrations. The apparatus includes a positive ion electrode (PIE) having a non-circular aperture specifically oriented with respect to the electron beam and a variable DC voltage source to apply a magnitude of DC voltage to the PIE. The PIE comprises a planar disk where the non-circular aperture is sized to permit passage of the electron beam through the aperture. The magnitude of the DC voltage, the aperture, and the alignment of the aperture with respect to the electron beam all serve to reduce variation in the spot size of the electron beam at the target.
Certain embodiments of the present invention afford an approach to reduce variation in the spot size of an electron beam of an EBT scanner due to multipole aberrations caused by the beam self-focusing forces.